Many types of electronic apparatus are known in which various electrical components are interconnected by conductors. The interconnecting conductors are fabricated in a wide variety of processes such as, for example, thick film fired conductor systems, polymer conductors and printed circuit boards. As the complexity of such apparatus (such as a circuit board and the like) increases, the requirement for the conductive layers to cross over one another and to make layer-to-layer connections also increases.
In thick-film fired conductors, a mixture of a conducting metal powder, a ceramic or glass binder and an appropriate vehicle is screen printed on a substrate. The conductor pattern on the substrate is then fired at a relatively high temperature, typically between 650.degree. and 900.degree. C. As the temperature rises to the firing temperature, the vehicle is volatilized leaving the metal and binder behind. At the firing temperature, sintering of the metal takes place to a greater or lesser extent with the binder providing adhesion between the metal film formed and the substrate.
Thick film fired conductors have classically employed precious metals such as gold, silver, platinum and palladium. Recently these noble metals have soared in cost, and new conductor systems using copper, nickel and aluminum are being made commercially available. The cost of the precious metal systems is prohibitive where a low cost conductor system is desired. The newer metal systems are not significantly cheaper because of the special chemistry which is required to prevent oxidation of the metal during the firing process. Moreover, these systems are very difficult to solder using the conventional tin/lead solder and the high firing temperatures required during fabrication preclude the use of low cost substrate materials. Some of the nickel systems can be fired on soda-lime glass at temperatures just below the melting point of the glass but the resulting conductivity of the conductor is relatively low.
Thick film conductor arrangements have been prepared in which a layer of the conductor is printed and then sintered, followed by applying a layer of a dielectric material which contains gaps or holes for making connections to the first layer of conductors and which is fired, a second layer of conductor is applied such that it overlays the holes, and contact is made between the first and second conductor layers. The second conductor layer is sintered to provide connection between the first and second conductor layer through the holes as desired. This technique, however, is undesirable because it is both costly and involves a number of complicated processing steps.
The term "polymer conductor" is actually a misnomer since the polymer is not actually a conductor. Instead, the polymer is heavily loaded with a conducting metal and screened onto a substrate. The advantage of this system is that the polymer can be cured either catalytically or thermally at temperatures which range from room temperature to about 125.degree. C. As a result of this so-called "cold processing", it is possible to use very inexpensive substrates such as films of Mylar.RTM. (polyethylene terephthalate). The mechanism by which conductivity is achieved is supplied entirely by contact between individual metallic particles. It has been found that the only metals which can be loaded into the polymer and give acceptable conductivity are the precious metals such as gold and silver. All of the other standard conducting metals oxidize over a period of time and the conductivity between the particles is reduced. Silver has been the predominant choice in polymer conductor systems but the silver systems are generally not solderable because the silver is leached by the lead-tin solder. When the price of silver is about $10-11 per ounce, these conductor systems are competitive with other systems if used on very low cost substrates such as thin mylar films. However, when the price of silver is higher, the systems are not competitive with printed circuit boards.
Multiple layer arrangements with conductor-to-conductor contact are achieved with polymer conductors in the same way as with the thick film conductors except that the polymer is cured rather than the film being sintered. An advantage of this process is that high temperature firing of the conductors and dielectric is not required but a problem which plagues this type of multilayer circuit is silver migration. The silver in the upper and lower levels tend to migrate through the dielectric making undesired connection between the two conductor layers. This process is accelerated by the presence of moisture and heat and by the application of high field voltages. This particular phenomenon is especially insidious because original quality control tests are passed and field failures later occur. Gold or palladium-silver loaded conductors can be used to eliminate the problem but such a solution makes the process no longer economically viable.
The techniques used to prepare printed circuit boards can be divided into additive and subtractive technologies. In both, the starting point is a substrate, which can vary widely from phenolics to glass filled epoxies, on which a copper foil is bonded. In the additive preparatory system, the copper foil is very thin, usually on the order of about 200 microinches. A resist is patterned such that the copper is exposed only where the conductors are desired and the board is then electroplated to form copper conductors of about 1 mil in thickness. The plating resist is stripped and the copper is etched. In areas where the conductor is not desired, the copper is only about 200 microinches thick so that etching quickly removes this copper while leaving a 1 mil thick conductor. In the subtractive process, the starting thickness of the copper foil is usually between 1 and 2 mils. An etch resist is deposited wherever the conductors are desired, the board is etched and the resist is then removed. The resist prevents etching where the conductors are desired leaving conductor runs.
Both the additive and subtractive printed circuit board procedures require the application of a copper foil over the entire substrate, deposition and removal of a resist, etching of the printed circuit board, drilling holes for component insertion, and in one case, the additional step of electroplating. An advantage of this technology is, however, that the resulting circuit boards can be relatively easily soldered.
Another advantage of the printed circuit board technology is that plated through holes can be fabricated. This process usually involves the addition of several steps to the additive fabrication process. Holes are drilled in the substrate before the resist is applied over all areas except where the conductors are desired. The board is then soaked in a stannous chloride sensitizer which forms a coating over the exposed parts of the substrate, namely inside the holes. The board is then sequentially dipped in a bath of palladium chloride, acid to dissolve the tin chloride, and an electroless copper bath. The last step, i.e., immersion in an electroless copper bath, deposits a very thin film of copper inside the activated hole. This "electroless copper" is plated out by a catalytic reaction in which the catalyst is copper such that a continuous plating reaction can occur. Typically, thickness on the order of 24-50 microinches can be achieved in 0.5 hour. At this point, a thin film of copper is adhered to the inside edges of the holes. The subsequent electroplating will build up the thickness of the copper within the holes as well as along all of the conductor runs. At this point, the various processes employed differ. The simplest process merely strips the resist and then etches, eliminating the much thinner copper were the conductor runs are not desired. In more complex processes, electroplating of tin-lead solder is accomplished which results in a tin-lead solder inside the hole and along the conductor runs, and is followed by etching with chromic acid which does not attack the tin-lead solder so that the solder acts as an etch resist.
The most significant drawback of the printed circuit board technology is that a substantial number of processing steps are necessary and this requires a large amount of associated equipment. In addition, the choice of substrate materials is limited to one of those available for circuit board materials. The number of process steps and equipment results in relatively high processing costs and the limitation of the substrate materials eliminates the opportunity to use a decorative or structural member, which may be required in the apparatus, as the substrate. Typical total costs for processed printed circuit boards range from $0.03 to $0.15 per square inch depending on the quality of the board and whether the board is single-sided or double-sided. Plated through holes require a large number of processing steps and the use of costly materials. Multilayer printed circuit boards presently range in cost from $0.10 to over $1.00 per square inch, depending on the number of layers and board quality factors, all of which cost is unacceptable in consumer electronic devices.
Another problem with the multilayer circuits where the dielectric material is applied by a printing technique such as screen printing, is that the viscosity of the dielectric material must be between 25 Kcps and 200 Kcps to permit such application while obtaining a suitable thickness to realize the desired insulator capacity. At such viscosities, applicator marks such as screen marks and small pin holes are formed which allow short circuits between conductor layers or to the substrate if formed of a conductor, such as steel. This problem is compounded by the fact that screen printing inks, especially those loaded with a filler, are thixotropic. Under the shear of the screening application, the ink viscosity is reduced so that it flows easily, but once in place, no shear forces are present, causing the imperfections in the dielectric to be retained. Heretofore, the standard method of improving dielectric strength was to print and cure/fire the dielectric twice, using different printing paths. While the desired results are achieved, additional labor and costs are involved.